1. Field of the Invention
The present invention relates to a system for thrust control of a gas turbine engine and, more particularly, to a system which provides a speed signal to the pilot that compensates for thrust performance variations in accordance with measured thrust performance characteristics of a particular gas turbine engine from the standard thrust performance characteristics of that engine model to enable the pilot to more accurately match engine thrust output to thrust requirements.
2. Discussion of the Background Art
Design, manufacture and control variations and accumulated tolerances of gas turbine engines cause variation in the operating characteristics from engine to engine within an engine model. One of these operating characteristics is thrust output. In order to provide safe operation of the aircraft, gas turbine engines are commonly designed to generate a predetermined minimum guaranteed or rated thrust level on any given engine regardless of variations in components and subsystems. In addition, pilots operate the aircraft according to predetermined engine model performance curves. These curves allow the pilot to set speed controls to provide the needed thrust for any engine of a model designation even if all accumulated variations in a particular engine are biased toward an engine configuration which produces minimum thrust output. Any engine having tolerances and variations, which are not limited to the least output configuration, will produce more thrust than can be effectively used by the aircraft under particular operating conditions. Excess thrust levels with accompanying higher operating temperatures result from higher than required engine rotational speed. The higher engine operating temperatures contribute to increased engine wear and deterioration which decrease the operating life of the engine.
In the Prior Art for gas turbine engines with hydromechanical fuel systems, the control architecture was designed such that the xe2x80x9cminimumxe2x80x9d or xe2x80x9cworst casexe2x80x9d engine performance produces the required thrust with margin at the target thrust setting parameter (fan speed). The target fan speed schedules are determined based on environmental conditions, engine performance characteristics and desired thrust rating. Due to the limitations in hydromechanical fuel systems, individual tailoring of the target fan speed schedules on an engine by engine basis has not been accomplished. Therefore, the thrust setting parameter schedule selected for the xe2x80x9cminimumxe2x80x9d or xe2x80x9cworst casexe2x80x9d engine performance produces a large population of engines with excess thrust.
The present invention is a rotational speed indication system for gas turbine engines with hydromechanical fuel controls that compensates for variations in thrust actually produced as compared to the nominal thrust. The invention uses predetermined engine model performance curves and actual engine performance data to calculate a modified thrust setting parameter (fan speed). Curves representing fan speed modification for a particular engine model are programmed into a memory device. Selection of a specific fan speed modification curve produces a speed indication that allows the pilot to more closely match engine thrust to a desired level of thrust. The generated output signal is then sent to the engine operator""s fan speed signal display unit where the modified signal is used by the pilot to set the engine fan speed which corresponds to the desired thrust level. More particularly, the engine operator sets the engine throttle so that the modified fan speed signal matches the target fan speed corresponding to the desired thrust rating for the given ambient conditions. Since the target fan speed is set using the modified fan speed signal, the engine will run at a lower speed as compared to a target set using the unmodified fan speed signal. This in turn results in the gas turbine engine producing the needed thrust while eliminating excess thrust margin caused by the engine-to-engine variation, thus reducing engine temperatures and improving engine operating life.